As the chill of winter approaches, there is a subtle change at your local gas station you may not notice—the introduction of winter-blend gasoline. You probably won’t see a change in how your car runs, but you might notice that it costs a little less for a fill-up. So what’s the difference between summer- and winter-blend gas, how does it affect your car’s engine, and why does it cost less? You’ve got questions, we’ve got answers.

What’s the difference between summer- and winter-blend gas?

Winter-blend gas has a higher Reid Vapor Pressure (RVP), which is a measure of fuel evaporation at a given temperature. This higher RVP is mostly due to a higher percentage of butane in the winter-blend fuel mixture. Butane—which you may also know as lighter fluid—has high volatility and vaporizes at lower temperatures, a helpful combination when you start your car’s engine on a cold winter morning.

Other additives in the winter-blend formulation are there to prevent fuel line freezing and improve engine performance when temperatures drop extremely low.

Why is winter-blend gas less expensive?

There are several factors that contribute to the lower cost of gas in winter, but the main reasons are lower production costs and the law of supply and demand. Butane and other winter-blend fuel additives are less expensive than gasoline’s summer-blend components, and those savings are usually passed along to consumers. And because people tend to drive less in colder weather, the demand for fuel goes down, lowering its cost even further. 

When does winter-blend gas start?

It varies from year to year, but the exact timing depends on location and local climate patterns. Typically, the transition from summer- to winter-blend gas occurs in September or October, beginning in northern states and gradually moving south. In spring, the process is reversed, with the transition from winter- summer-blend formulation starting in the south and moving north.

Why can’t we use winter-blend gas all year?

Butane is less expensive than other gasoline components, but the reason it benefits your car’s engine in winter months is why it hinders engine efficiency in the summer. In warmer weather, butane’s high volatility generates excessive evaporation, which can cause vapor lock in an engine while also contributing to ground-level ozone and smog. 

Which is better for my car’s engine?

Although you can use both summer- and winter-blend gas any time of the year without harming your engine, their respective formulations are ideal for the corresponding season. Additionally, a higher butane concentration lowers fuel efficiency, as the winter blend contains fewer energy units per gallon than other gasoline components. In other words, the miles per gallon are slightly lower when using winter-blend gas.

The only thing you might notice as gas stations transition from summer- to winter-blend fuel is a slightly lower cost to fill up your car’s gas tank. But the science backs up its importance as temperatures drop and your car engine’s needs change.